The present invention relates to the field of consumer electronic product safety. More specifically, the invention relates to apparatus for detecting faults in a CATV coaxial cable carrying both RF signals and AC powering voltage to circuits and devices at subscriber premises, and for interrupting the circuit in response to such faults.
Since the advent of community antenna television (CATV) many homes have been connected by a coaxial drop cable to an outside pedestal or other device where the cable receives RF broadcast signals. Typically, the drop cable is buried a few inches below the ground surface to protect it from lawnmowers, foot traffic, etc. More recently, with deregulation of the cable television and telephone industries, it has become feasible to provide two-way data and telephone services through the existing coaxial cables. The electronic circuits which separate and direct the television, data and telephone signals, as well as the telephone circuits and sets themselves, require electrical power for operation. The typical CATV distribution system is passive at the subscriber end, i.e., it does not provide power for operation of customer equipment.
Although power could be derived from the AC power supply at the subscriber end, this has the disadvantage of connection to mains power and, since telephony is regarded as a life-line service, the need for battery backup in case of power failure. Thus, it is economically preferable to use the existing drop cables to provide the home circuit powering. This raises another problem due to the fact that the cables are not buried deep enough to provide the necessary degree of safety for the level of power (typically, 90V AC) carried thereby. In order to avoid the relatively costly task of re-burying or replacing existing drop cables, it is preferable to provide apparatus which will detect an open circuit or faulted condition and disconnect the cable from the power source before causing significant equipment damage or personal injury.
The present invention provides novel and improved apparatus for detecting and responding to fault conditions in a powered CATV coaxial cable.
Prior art systems such as that disclosed in U.S. Pat. No. 5,793,590 provide protection systems based on monitoring the electrical resistance between the inner and outer cables. The present invention provides a protective circuit operable in response to detected changes in the current on either the inner or outer conductors of the coaxial cable. The change in current may be measured by any of a number of means, such as monitoring the level of magnetic flux generated from the current. Alternatively, as in the disclosed embodiment, a resistor of known value is placed in the line and the voltage drop across the resistor is measured, the current being calculated from Ohm""s Law. In the event the cable is cut, the current on the powered conductor will approach zero. If the cable inner and outer conductors become electrically shorted, the current approaches infinity. In either case, the protective system of the invention triggers an electronic switch to open the circuit within an acceptably short time period.
The foregoing and other features of construction and operation of the invention will be more readily understood and fully appreciated from the following detailed disclosure, taken in conjunction with the accompanying drawings.